Add cache to `getSimplifiedConditionalType` by ahejlsberg · Pull Request #64648 · microsoft/TypeScript

Anders Hejlsberg (ahejlsberg) · 2026-10-05T19:34:45Z

With this PR we consistently cache results in getSimplifiedType. Previously, we cached results for indexed access types but not conditional types. Found in performance experiments by Bun team.

Anders Hejlsberg (ahejlsberg) · 2026-10-05T19:35:02Z

TypeScript Bot (@typescript-bot) test it

typescript-automation · 2026-10-05T19:35:08Z

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
`test top400`	✅ Started	✅ Results
`user test this`	✅ Started	✅ Results
`run dt`	✅ Started	✅ Results
`perf test this faster`	✅ Started	👀 Results

typescript-automation · 2026-10-05T19:58:11Z

Hey Anders Hejlsberg (@ahejlsberg), the results of running the DT tests are ready.

Everything looks the same!

You can check the log here.

Copilot

Copilot review overview

🟡 Changes recommended

The cache must distinguish read and write simplification modes to avoid reusing incorrect results.

Review effort: Balanced
Findings: 1

Open (1)

Separate cache keys for read and write simplification · New

What changed in this PR

Adds caching for simplified conditional types to improve checker performance.

Changes:

Adds a conditional-type cache kind.
Splits cached orchestration from simplification logic.

File	Description
`tsc/internal/checker/checker.go`	Caches conditional-type simplification results.

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Mateusz Burzyński (Andarist) · 2026-10-06T06:42:12Z

 }

 func (c *Checker) getSimplifiedConditionalType(t *Type, writing bool) *Type {
+	key := CachedTypeKey{kind: CachedTypeKindSimplifiedConditionalType, typeId: t.id}


regression tests for the above:

type Value<T extends { a: string; b: number }> = T["a" | "b"] extends unknown ? T["a" | "b"] : never; function test<T extends { a: string; b: number }>(value: Value<T>, a: T["a"]) { const read: T["a"] | T["b"] = value; const write: Value<T> = a; // errors }

Implemented separate read/write cache kinds and added regression coverage. Commit: dbb752eff.

typescript-automation · 2026-10-05T20:04:18Z

Anders Hejlsberg (@ahejlsberg)
The results of the perf run you requested are in!

Here they are:

tsc

Comparison Report - baseline..pr

_Metric	_baseline	_pr	_Delta	_Best	_Worst	_p-value
_{Compiler-Unions - native}
_Errors	₄₁	₄₁	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_115,276	_115,276	_~	_~	_~	_{p=1.000 n=12}
_Types	_79,613	_79,613	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{141,886k (± 0.52%)}	_{141,762k (± 0.38%)}	_~	_140,749k	_143,935k	_{p=0.590 n=12}
_{Memory Allocs}	_{2,193,643 (± 0.01%)}	_{2,193,559 (± 0.01%)}	_~	_2,193,188	_2,193,961	_{p=0.478 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.041s (± 3.00%)}	_{0.042s (± 3.35%)}	_~	_0.039s	_0.046s	_{p=0.612 n=12}
_{Bind Time}	_{0.012s (±14.80%)}	_{0.012s (± 9.22%)}	_~	_0.010s	_0.016s	_{p=0.603 n=12}
_{Check Time}	_{0.497s (± 0.86%)}	_{0.495s (± 0.61%)}	_~	_0.487s	_0.502s	_{p=0.448 n=12}
_{Emit Time}	_{0.272s (± 1.97%)}	_{0.267s (± 2.00%)}	_~	_0.248s	_0.279s	_{p=0.325 n=12}
_{Total Time}	_{0.829s (± 0.88%)}	_{0.823s (± 0.76%)}	_~	_0.808s	_0.841s	_{p=0.163 n=12}
_{angular-1 - native}
_Errors	₃	₃	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_{847,041 (± 0.06%)}	_{847,021 (± 0.07%)}	_~	_845,274	_848,203	_{p=0.887 n=12}
_Types	_{249,941 (± 0.00%)}	_{249,941 (± 0.00%)}	_~	_249,937	_249,950	_{p=0.444 n=12}
_{Memory Used}	_{789,597k (± 0.08%)}	_{789,537k (± 0.09%)}	_~	_787,622k	_791,463k	_{p=0.977 n=12}
_{Memory Allocs}	_{12,867,863 (± 0.07%)}	_{12,865,722 (± 0.06%)}	_~	_12,854,690	_12,892,669	_{p=0.590 n=12}
_{Config Time}	_{0.017s (± 1.95%)}	_0.017s	_{+0.000s (+ 2.51%)}	_~	_~	_{p=0.037 n=12}
_{Parse Time}	_{0.255s (± 3.01%)}	_{0.259s (± 2.28%)}	_~	_0.239s	_0.269s	_{p=0.325 n=12}
_{Bind Time}	_{0.066s (±17.51%)}	_{0.061s (±11.68%)}	_{🟩-0.005s (- 7.57%)}	_0.057s	_0.097s	_{p=0.029 n=12}
_{Check Time}	_0s	_0s	_~	_~	_~	_{p=1.000 n=12}
_{Emit Time}	_{1.601s (± 0.96%)}	_{1.593s (± 0.74%)}	_~	_1.565s	_1.630s	_{p=0.561 n=12}
_{Total Time}	_{1.951s (± 1.09%)}	_{1.941s (± 0.71%)}	_~	_1.906s	_1.995s	_{p=0.765 n=12}
_{mui-docs - native}
_Errors	_11,405	_11,405	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_4,351,480	_4,351,480	_~	_~	_~	_{p=1.000 n=12}
_Types	_1,380,970	_1,380,970	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{2,975,362k (± 0.04%)}	_{2,974,730k (± 0.04%)}	_~	_2,970,822k	_2,978,067k	_{p=0.671 n=12}
_{Memory Allocs}	_{32,840,597 (± 0.05%)}	_{32,787,989 (± 0.05%)}	_{-52,608 (- 0.16%)}	_32,750,727	_32,825,128	_{p=0.000 n=12}
_{Config Time}	_{0.017s (± 1.86%)}	_{0.017s (± 1.70%)}	_~	_0.016s	_0.017s	_{p=1.000 n=12}
_{Parse Time}	_{0.504s (± 2.33%)}	_{0.500s (± 2.13%)}	_~	_0.471s	_0.523s	_{p=0.660 n=12}
_{Bind Time}	_0.002s	_0.002s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{8.504s (± 0.87%)}	_{8.481s (± 0.72%)}	_~	_8.289s	_8.608s	_{p=0.504 n=12}
_{Emit Time}	_{0.440s (± 7.01%)}	_{0.434s (± 3.42%)}	_~	_0.419s	_0.496s	_{p=0.221 n=12}
_{Total Time}	_{10.228s (± 0.56%)}	_{10.203s (± 0.42%)}	_~	_10.084s	_10.322s	_{p=0.401 n=12}
_{strada-build-src - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,390,229	_1,390,229	_~	_~	_~	_{p=1.000 n=12}
_Types	_441,498	_441,498	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{1,694,289k (± 1.04%)}	_{1,689,782k (± 1.03%)}	_~	_1,648,439k	_1,724,345k	_{p=0.755 n=12}
_{Memory Allocs}	_{92,233,428 (± 0.10%)}	_{92,212,096 (± 0.17%)}	_~	_91,795,070	_92,518,137	_{p=0.713 n=12}
_{Config Time}	_{0.003s (±14.26%)}	_{0.003s (±12.31%)}	_~	_0.002s	_0.004s	_{p=0.496 n=12}
_{Parse Time}	_{0.197s (± 4.94%)}	_{0.203s (± 3.69%)}	_~	_0.184s	_0.218s	_{p=0.325 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.810s (± 0.59%)}	_{1.803s (± 0.61%)}	_~	_1.775s	_1.836s	_{p=0.340 n=12}
_{Emit Time}	_{0.285s (± 4.91%)}	_{0.303s (± 5.69%)}	_~	_0.261s	_0.351s	_{p=0.075 n=12}
_{Total Time}	_{23.396s (± 0.79%)}	_{23.572s (± 0.78%)}	_~	_23.182s	_24.077s	_{p=0.219 n=12}
_{strada-compiler - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_334,953	_334,953	_~	_~	_~	_{p=1.000 n=12}
_Types	_197,616	_197,616	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{313,310k (± 0.03%)}	_{313,334k (± 0.03%)}	_~	_313,130k	_313,597k	_{p=0.843 n=12}
_{Memory Allocs}	_{4,623,066 (± 0.01%)}	_{4,622,687 (± 0.02%)}	_~	_4,620,003	_4,623,784	_{p=0.443 n=12}
_{Config Time}	_0.001s	_0.001s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.115s (± 5.89%)}	_{0.113s (± 5.28%)}	_~	_0.102s	_0.135s	_{p=0.619 n=12}
_{Bind Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Check Time}	_{1.050s (± 0.49%)}	_{1.044s (± 0.41%)}	_~	_1.033s	_1.056s	_{p=0.060 n=12}
_{Emit Time}	_{0.135s (±11.97%)}	_{0.130s (±12.31%)}	_~	_0.091s	_0.165s	_{p=0.504 n=12}
_{Total Time}	_{1.351s (± 1.26%)}	_{1.339s (± 1.03%)}	_~	_1.306s	_1.387s	_{p=0.139 n=12}
_{ts-pre-modules - native}
_Errors	₈₇	₈₇	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_303,834	_303,834	_~	_~	_~	_{p=1.000 n=12}
_Types	_181,663	_181,663	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{273,915k (± 0.02%)}	_{273,909k (± 0.04%)}	_~	_273,417k	_274,129k	_{p=0.660 n=12}
_{Memory Allocs}	_{1,615,853 (± 0.02%)}	_{1,615,912 (± 0.01%)}	_~	_1,615,351	_1,616,485	_{p=0.799 n=12}
_{Config Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.099s (± 5.64%)}	_{0.100s (± 5.26%)}	_~	_0.090s	_0.112s	_{p=0.744 n=12}
_{Bind Time}	_{0.036s (± 8.36%)}	_{0.036s (± 8.36%)}	_~	_0.030s	_0.045s	_{p=0.898 n=12}
_{Check Time}	_{0.823s (± 0.52%)}	_{0.824s (± 1.07%)}	_~	_0.811s	_0.853s	_{p=0.701 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{0.973s (± 0.80%)}	_{0.974s (± 1.31%)}	_~	_0.950s	_1.014s	_{p=0.899 n=12}
_{vscode - native}
_Errors	₃₈₀	₃₈₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_10,591,714	_10,591,714	_~	_~	_~	_{p=1.000 n=12}
_Types	_3,537,253	_3,537,253	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{6,870,134k (± 0.02%)}	_{6,868,705k (± 0.01%)}	_~	_6,866,420k	_6,870,554k	_{p=0.052 n=12}
_{Memory Allocs}	_{52,733,074 (± 0.02%)}	_{52,724,484 (± 0.03%)}	_{-8,590 (- 0.02%)}	_52,707,198	_52,793,992	_{p=0.045 n=12}
_{Config Time}	_{0.073s (± 0.37%)}	_{0.073s (± 0.50%)}	_~	_0.072s	_0.074s	_{p=0.496 n=12}
_{Parse Time}	_{1.808s (± 3.58%)}	_{1.775s (± 3.75%)}	_~	_1.621s	_1.926s	_{p=0.340 n=12}
_{Bind Time}	_{0.547s (±20.58%)}	_{0.556s (±20.58%)}	_~	_0.428s	_0.901s	_{p=0.560 n=12}
_{Check Time}	_{13.323s (± 1.27%)}	_{13.280s (± 1.22%)}	_~	_12.936s	_13.584s	_{p=0.799 n=12}
_{Emit Time}	_{4.636s (±10.41%)}	_{5.155s (± 9.81%)}	_~	_3.916s	_5.986s	_{p=0.219 n=12}
_{Total Time}	_{20.467s (± 1.85%)}	_{20.922s (± 1.82%)}	_~	_19.953s	_21.570s	_{p=0.060 n=12}
_{webpack - native}
_Errors	₅₈₉	₅₈₉	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,303,232	_1,303,232	_~	_~	_~	_{p=1.000 n=12}
_Types	_618,887	_618,887	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{1,005,469k (± 0.02%)}	_{1,005,584k (± 0.01%)}	_~	_1,005,244k	_1,005,927k	_{p=0.551 n=12}
_{Memory Allocs}	_{6,545,098 (± 0.03%)}	_{6,539,428 (± 0.02%)}	_{-5,670 (- 0.09%)}	_6,536,137	_6,543,213	_{p=0.000 n=12}
_{Config Time}	_0.009s	_0.009s	_~	_~	_~	_{p=1.000 n=12}
_{Parse Time}	_{0.282s (± 3.42%)}	_{0.281s (± 2.43%)}	_~	_0.263s	_0.307s	_{p=0.723 n=12}
_{Bind Time}	_{0.085s (±20.63%)}	_{0.084s (±14.27%)}	_~	_0.063s	_0.120s	_{p=0.680 n=12}
_{Check Time}	_{1.991s (± 0.58%)}	_{1.980s (± 0.92%)}	_~	_1.939s	_2.022s	_{p=0.325 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{2.400s (± 0.31%)}	_{2.393s (± 0.62%)}	_~	_2.341s	_2.422s	_{p=0.921 n=12}
_{xstate-main - native}
_Errors	₀	₀	_~	_~	_~	_{p=1.000 n=12}
_Symbols	_1,055,167	_1,055,167	_~	_~	_~	_{p=1.000 n=12}
_Types	_387,939	_387,939	_~	_~	_~	_{p=1.000 n=12}
_{Memory Used}	_{610,319k (± 0.02%)}	_{610,351k (± 0.03%)}	_~	_609,818k	_610,905k	_{p=0.831 n=12}
_{Memory Allocs}	_{4,880,869 (± 0.14%)}	_{4,873,857 (± 0.08%)}	_~	_4,864,898	_4,886,162	_{p=0.143 n=12}
_{Config Time}	_{0.003s (± 8.64%)}	_{0.003s (±10.34%)}	_~	_0.002s	_0.003s	_{p=1.000 n=12}
_{Parse Time}	_{0.143s (± 2.88%)}	_{0.145s (± 3.16%)}	_~	_0.134s	_0.160s	_{p=0.541 n=12}
_{Bind Time}	_{0.043s (±19.29%)}	_{0.041s (±14.82%)}	_~	_0.032s	_0.061s	_{p=0.853 n=12}
_{Check Time}	_{1.291s (± 0.52%)}	_{1.295s (± 1.01%)}	_~	_1.272s	_1.343s	_{p=0.944 n=12}
_{Emit Time}	_0.000s	_0.000s	_~	_~	_~	_{p=1.000 n=12}
_{Total Time}	_{1.489s (± 0.54%)}	_{1.494s (± 0.78%)}	_~	_1.474s	_1.543s	_{p=0.702 n=12}

System info unknown

Hosts

native

Scenarios

Compiler-Unions - native
angular-1 - native
mui-docs - native
strada-build-src - native
strada-compiler - native
ts-pre-modules - native
vscode - native
webpack - native
xstate-main - native

_Benchmark	_Name	_Iterations
_Current	_pr	₁₂
_Baseline	_baseline	₁₂

Developer Information:

Download Benchmarks

typescript-automation · 2026-10-05T20:07:13Z

Anders Hejlsberg (@ahejlsberg) Here are the results of running the user tests with tsc comparing baseline and pr:

Everything looks good!

typescript-automation · 2026-10-05T20:54:28Z

Anders Hejlsberg (@ahejlsberg) Here are the results of running the top 400 repos with tsc comparing baseline and pr:

Everything looks good!

Anders Hejlsberg (ahejlsberg) · 2026-10-05T22:50:50Z

No measurable effect on perf tests, likely because we don't have react tests which is where it's supposed to help.

Co-authored-by: ahejlsberg <4226954+ahejlsberg@users.noreply.github.com>

Jake Bailey (jakebailey)

Still want this?

Add cache to getSimplifiedConditionalType

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Separate conditional simplification cache modes

dbb752e

Co-authored-by: ahejlsberg <4226954+ahejlsberg@users.noreply.github.com>

Copilot finished work on behalf of Anders Hejlsberg (ahejlsberg) October 8, 2026 21:18

Jake Bailey (jakebailey) approved these changes Oct 13, 2026

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Merge branch 'main' into cacheSimplifiedConditionalTypes

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Merged via the queue into main with commit 6aace8b Oct 14, 2026
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Add cache to `getSimplifiedConditionalType` - #64648

Add cache to `getSimplifiedConditionalType`#64648
Anders Hejlsberg (ahejlsberg) merged 3 commits into
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cacheSimplifiedConditionalTypes

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